1. Field of the Invention
The present invention relates to isolated polypeptides having phytase activity and isolated nucleic acid sequences encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the nucleic acid sequences as well as methods for producing the polypeptides. The invention further relates to compositions comprising the polypeptides and methods of use thereof.
2. Description of the Related Art
Phytases (myo-inositol hexakisphosphate phosphohydrolases, EC 3.1.3.8) catalyze the hydrolysis of phytate (myo-inositol hexakisphosphate) to (1) myo-inositol, (2) mono-, di-, tri-, tetra- and penta-phosphates thereof and (3) inorganic phosphate. In the following, for short, the above compounds are sometimes referred to as IP6, I, IP1, IP2, IP3, IP4, IP5 and P, respectively. This means that by action of a phytase, IP6 is degraded into inorganic phosphate and one or more of the components IP5, IP4, IP3, IP2, IP1 and I. Alternatively, myo-inositol carrying in total n phosphate groups attached to positions p, q, r, . . . is denoted (Ins(p,q,r, . . .) P.sub.n).
Two different types of phytases are known: A so-called 3-phytase (myo-inositol hexakisphosphate 3-phosphohydrolase, EC 3.1.3.8) and a so-called 6-phytase (myo-inositol hexakisphosphate 6-phosphohydrolase, EC 3.1.3.26). The 3-phytase hydrolyzes first the ester bond at the 3-position, whereas the 6-phytase hydrolyzes first the ester bond at the 6-position. The remaining ester bonds of the resulting IP5 substrate (whether the 1,2,4,5,6-IP5 or the 1,2,3,4,5-IP5) are subsequently hydrolyzed at different rates. Also the rate of hydrolysis of the components IP4, IP3, IP2 and IP1 seems to be variable, if hydrolyzed at all.
Phytase-producing microorganisms include bacteria such as Bacillus subtilis (Paver and Jagannathan, 1982, Journal of Bacteriology 151: 1102-1108) and Pseudomonas (Cosgrove, 1970, Australian Journal of Biological Sciences 23: 1207-1220); yeast such as Saccharomyces cerevisiae (Navini and Marcakis, 1984, Lebensmittei Wissenschaft und Technologie 17: 24-26; and fungi of the Aspergillus genus such as Aspergillus terreus (Yamada et al., 1986, Agricultural Biological Chemistry 322: 1275-1282).
The cloning and expression of the phytase genes from Aspergillus niger var. awamori by Piddington et al. (1993, Gene 133: 55-62) and Aspergillus niger (ficuum) by van Hartingsveldt et al. (1993, Gene 127: 87-94; EP 420 358) have been disclosed.
Phytic acid is the primary storage form of phosphate in cereal grains, legumes, and oilseeds, such as soy, which are the principal components of animal feeds. However, the presence of phytic acid in animal feeds for monogastric animals is undesirable because the phosphate moieties of phytic acid chelate essential minerals and possibly proteins making them nutritionally unavailable. Furthermore, phytate phosphorus passes through the gastrointestinal tract of monogastric animals and is not metabolized. Since phosphorus is an essential element for the growth of all organisms, livestock feed must be supplemented with inorganic phosphate. Thus, the art has described the use of phytases in feeds of monogastric animals.
Furthermore, since phytic acid is not metabolized by monogastric animals, it is excreted in manure. The amount of manure produced worldwide has increased significantly resulting from increased livestock production. The disposal of manure has caused an environmental problem in various locations around the world due to the accumulation of phosphate particularly in water. Thus, the art has also described the use of phytases for reducing the amount of phytate in manure.
There is a need in the art for new phytases with improved properties which can be produced in commercially significant quantities.
It is an object of the present invention to provide a new class of phytases, i.e., 3,6-phytases, i.e., phytases which attack both bonds of a phosphoester.